A semi-closed loop-type control circuit of the kind shown in FIG. 3 is employed as a CNC servo-control circuit. In the Figure, an information processing circuit X comprises a CPU and a memory and forms a command signal for a servomotor. A servomechanism Y is constituted by a comparator circuit (a) for comparing the command signal and a signal from a position detector e, a servo drive circuit b, a servomotor c, a velocity detector d and a position detector e employing a resolver or a pulse encoder, and is adapted to control a machine Z such as a table via a ball screw shaft f.
As illustrated, the semi-closed loop system controls a load by performing position detection at a portion of the motor shaft or ball screw shaft in front to be finally controlled. Control is performed accurately up to the motor shaft or ball screw shaft, and from this point onward control depends on the precision of the machine.
FIG. 4 is a block diagram of such a servo-control system, in which K denotes position gain, k.sub.1 and k.sub.2 gains, K.sub.T a torque constant, T.sub.L disturbance, and J.sub.m rotor inertia.
In control of the machine by such a semi-closed loop system, a response delay due to backlash in the mechanical system arises when there is a change in the direction of servomotor rotation. In order to improve upon this delay with respect to a move command, the conventional practice is to supply the servo system with a backlash correction signal together with the move command signal.
FIG. 2 is a block diagram illustrating an example of a control circuit in which a backlash correction signal BL(i) of this kind is inputted together with a move command signal MC(i) to the servo system to form a torque command. In FIG. 2, an integration term Z.sup.-1 is expressed using a Z conversion (pulse transfer function conversion). VEL(i) indicates a fed back velocity signal, and POS(i) represents an amount of movement in a sampling period T, namely a fed back position signal.
In this conventional control circuit, the response delay of the servo system is improved by inputting the backlash correction signal to the servo system. However, since frictional resistance is present in an actual servomotor and machine, positional displacement based on a response delay still remains when the servomotor reverses its direction of rotation. If a machine tool is to be controlled along, e.g., a circular arc by controlling a plurality of axes simultaneously, positional displacement caused by frictional resistance in the machine tool will hinder machining along a true circle.